1. Field of the Invention
This invention relates generally to chemical mechanical planarization apparatuses, and more particularly to methods and apparatuses for improved uniformity in chemical mechanical planarization applications via platen pressure zones outside the wafer's area.
2. Description of the Related Art
In the fabrication of semiconductor devices, there is a need to perform chemical mechanical planarization (CMP) operations. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. As is well known, patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material grows. Without planarization, fabrication of further metallization layers becomes substantially more difficult due to the variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then, metal CMP operations are performed to remove excess material.
A chemical mechanical planarization (CMP) system is typically utilized to polish a wafer as described above. A CMP system typically includes system components for handling and polishing the surface of a wafer. Such components can be, for example, an orbital polishing pad, or a linear belt polishing pad. The pad itself is typically made of a polyurethane material or polyurethane in conjunction with other materials such as, for example a stainless steel belt. In operation, the belt pad is put in motion and then a slurry material is applied and spread over the surface of the belt pad. Once the belt pad having slurry on it is moving at a desired rate, the wafer is lowered onto the surface of the belt pad. In this manner, wafer surface that is desired to be planarized is substantially smoothed, much like sandpaper may be used to sand wood. The wafer may then be cleaned in a wafer cleaning system.
FIG. 1A shows a linear polishing apparatus 10 which is typically utilized in a CMP system. The linear polishing apparatus 10 polishes away materials on a surface of a semiconductor wafer 16. The material being removed may be a substrate material of the wafer 16 or one or more layers formed on the wafer 16. Such a layer typically includes one or more of any type of material formed or present during a CMP process such as, for example, dielectric materials, silicon nitride, metals (e.g., aluminum and copper), metal alloys, semiconductor materials, etc. Typically, CMP may be utilized to polish the one or more of the layers on the wafer 16 to planarize a surface layer of the wafer 16.
The linear polishing apparatus 10 utilizes a polishing belt 12, which moves linearly in respect to the surface of the wafer 16. The belt 12 is a continuous belt rotating about rollers (or spindles) 20. A motor typically drives the rollers so that the rotational motion of the rollers 20 causes the polishing belt 12 to be driven in a linear motion 22 with respect to the wafer 16.
A wafer carrier 18 holds the wafer 16. The wafer 16 is typically held in position by mechanical retaining ring and/or by vacuum. The wafer carrier positions the wafer atop the polishing belt 12 so that the surface of the wafer 16 comes in contact with a polishing surface of the polishing belt 12.
FIG. 1B shows a side view of the linear polishing apparatus 10. As discussed above in reference to FIG. 1A, the wafer carrier 18 holds the wafer 16 in position over the polishing belt 12 while applying pressure to the polishing belt. The polishing belt 12 is a continuous belt typically made up of a polymer material such as, for example, the IC 1000 made by Rodel, Inc. layered upon a supporting layer. The polishing belt 12 is rotated by the rollers 20 which drives the polishing belt in the linear motion 22 with respect to the wafer 16. In one example, a fluid bearing platen 24 supports a section of the polishing belt under the region where the wafer 16 is applied. The platen 24 can then be used to apply fluid against the under surface of the supporting layer. The applied fluid thus forms a fluid bearing that creates a polishing pressure on the underside of the polishing belt 12 which is applied against the surface of the wafer 16. Unfortunately, because the polishing pressure produced by the fluid bearing typically cannot be controlled very well, the polishing pressure applied by the fluid bearing to different parts of the wafer 16 generally is non-uniform. Generally, uniformity requires all parameters defining the material removal rate to be evenly distributed across the entire contact surface that interfaces with the wafer.
Edge instabilities in CMP are among the most significant performance affecting issues and among the most complicated problems to resolve. FIG. 1C shows a linear polishing apparatus 10 illustrating edge effect non-uniformity factors. In this example, a wafer 16 is attached to a carrier 18, which applies pressure 13 to push the wafer 16 down on the polishing belt 12 that is moving over the platen 24. However, the polishing belt 12 deforms when the wafer contacts the polishing belt 12. Although the polishing belt 12 is a compressible medium, the polishing belt 12 has limited flexibility, which prevents the polishing belt 12 from conforming to the exact shape of the wafer 16, forming transient deformation zones 22 and 26. As a result, edge effects occur at the wafer edge 16a and 16b from a non-flat contact field resulting from redistributed contact forces. Hence, large variations in removal rates occur at the wafer edge 16a and 16b. Consequently, due to the fact that the prior art polishing belt designs do not properly control polishing dynamics, uneven polishing and inconsistent wafer polishing may result thereby decreasing wafer yield and increasing wafer costs.
In view of the foregoing, there is a need for an apparatus that overcomes the problems of the prior art by having a platen that improves polishing pressure control and reduces polishing pad deformation.